Many researches and developments are actively made on planar lightwave circuits (PLCs) each including silica glass waveguides formed on a silicon substrate. Arrayed waveguide gratings (AWGs) using such PLC techniques play an important role in optical communication systems as optical wavelength multi/demultiplexers having a function of demultiplexing a multiplexed light signal having plural optical frequencies (a wavelength division multiplexed signal) into light signals with a predetermined optical frequency channel spacing or a function of multiplexing light signals into a single wavelength division multiplexed signal.
On the other hand, with the progress in optical communication systems, systems connecting plural points and flexibly switching communication paths, such as ring and mesh networks, are being constructed. In such an advanced network, light signals are required to pass through a number of points without being demodulated into electrical signals, and the optical wavelength multi/demultiplexer used in such a system is required to have a broad and flat passband. Such optical wavelength multi/demultiplexers of flat transmission characteristics which have been proposed are a parabola AWG-type optical wavelength multi/demultiplexer including a parabola waveguide at an input end of a slab waveguide and an MZI-synchronized AWG optical wavelength multi/demultiplexer including a combination of a Mach-Zehnder Interferometer (MZI) and an AWG. These related arts are disclosed in PTLs 1 and 2 in detail.
FIG. 1 illustrates an example of the configuration of a parabola AWG type wavelength multi/demultiplexer. A parabola AWG-type wavelength multi/demultiplexer 100 includes a first slab waveguide 101, an arrayed-waveguide 102, a second slab waveguide 103, output waveguides 104, an input waveguide 105, and a parabola waveguide 106. Light incoming through the input waveguide 105 passes through the parabola waveguide 106 and then has a bimodal electric field distribution as shown in FIG. 2. The light having the thus-obtained electric field distribution diffracts and passes through the first slab waveguide 101, and is then excited and propagates through each waveguide of the arrayed-waveguide 102. The light is focused at positions of the output waveguides 104 according to the optical frequencies in the second slab waveguide 103. Herein, the electric field distribution of the light focused at the interface between the slab waveguide 103 and the output waveguides 104 is also bimodal due to the reciprocity theorem. On the other hand, the electric field distribution of the output waveguides 104 receiving the light is a Gaussian distribution as shown in FIG. 2 and has a small width. Accordingly, even if the optical frequency of the incoming light change to shift the light-focusing positions, the overlap integral of both the electric field distributions is kept constant, and thus flat transmission characteristics are obtained as shown in FIG. 3A. Herein, FIG. 3A shows a case of a channel spacing of 100 GHz (0.8 nm). FIG. 3B is an enlarged view of FIG. 3A.
FIG. 4 illustrates an example of the configuration of an MZI-synchronized AWG type wavelength multi/demultiplexer. An MZI-synchronized AWG type wavelength multi/demultiplexer 400 includes a first slab waveguide 401, an arrayed-waveguide 402, a second slab waveguide 403, output waveguides 404, an input waveguide 405, a first optical coupler 406, a first arm waveguide 407, a second arm waveguide 408, and a second optical coupler 409. Light incoming through the input waveguide 405 is distributed to the first and second arm waveguides 407 and 408 by the second optical coupler 409, and thus the light beams have a phase difference according to the optical frequencies due to the optical path difference between the waveguides 407 and 408. The light beams traveling the arm waveguides are combined by the first optical coupler 406 to interfere with each other. The light focusing positions at the interface between the first optical coupler 406 and first slab waveguide 401 periodically changes according to the phase differences (or optical frequencies) between the light beams as shown in FIGS. 5A and 5B. The light entering the first slab waveguide 401 from the first optical coupler 406 diffracts and is excited to propagate through each waveguide of the arrayed-waveguide 402. The light focusing positions in the second slab waveguide 403 vary according to the optical frequencies, and the light beams having desired optical frequencies are distributed to the respective output waveguides 404.
Next, a description is given of the principle to obtain the flat transmission characteristics. When the optical frequency changes from a lower frequency than the central frequency to a higher one, the coordinate of the incident position of the optical field distribution output from the MZI circuit changes from a negative value to a positive value at the interface between the first optical coupler 406 and the first slab waveguide 401. If the change in optical frequency is neglected, the coordinate of the position at which the light beams are focused at the interface between the second slab waveguide 403 and output waveguide 404 changes from a positive value to a negative value due to the optical path differences in the arrayed-waveguide 402. Moreover, when the optical frequency changes from a lower frequency than the central frequency to a higher one, the coordinate of the position at which the light beams are focused at the interface between the second slab waveguide 403 and the output waveguide 404 change from a negative value to a positive value due to the path length difference. The effects of both the cases are thus cancelled out with each other, and the light focusing position remains unchanged even if the optical frequency changes. The flat transmission characteristics shown in FIGS. 6A and 6B can be obtained. Herein, the channel spacing in the drawings is 100 GHz (0.8 nm).